The round starts with the attacker units fireing (if they are not on the frontline) at random enemy groups, after that the defender ranged units fire back.

Once the ranged phase is over the attacker units on the frontline will start engaging the enemy units that are in the slot in front of them. The maximum number of units that can engage from a single group is 100, so it's a good rule to follow during the forming of a formation to never have groups of less than 100 units to not be at a numerical disadvantage during battle. If an attacking group doesn't have a group in front of them to engage it will engage the nearest enemy group it can find, but this time without retaliation from the enemy group (advantage of being surrounded). This means that having a more spread out frontline can be an advantage, however it's never good to dillute your troops, which means never have groups of less than 100 units.

Once the melee phase is over if neither side lost all its units another round will start but this time the attacker and the defender will switch roles.

The battle mechanics during a city battle are a little different and must tak into account the advantage that city walls give to the defender. You can read about them in the city battle section.